Various types of terminals including information home appliances are connected to a network due to the full-scale implementation of the Next Generation Network (NGN) as a communication system. This requires further expansion in the transmission capacity of the network. In the Non-Return-to-Zero (NRZ) modulation method, the transmission capacity is approaching the limit in terms of the usage efficiency of the optical frequency and the like. Thus, a modulation method utilizing an optical phase is under review when upgrading a long distance dense wavelength division modulation (DWDM) transmission system from 10 Gbps to 40 Gbps. The Quadrature Phase Shift Key (QPSK) modulation method and the Differential Quadrature Phase Shift Key (DQPSK) modulation method, which relate 2-bit information to four phases (e.g. 0, π/2, π and 3π/2), achieve high usage efficiency of the optical frequency, and attract the attention as the transmission method for 40 Gbps and 100 Gbps.
When the optical signal is received in the QPSK modulation method or the DQPSK modulation method, the optical phase modulation signal is separated into the I-phase signal and the Q-phase signal. The I-phase signal is delayed by −π/4,the Q-phase signal is delayed by π/4, and they are made to interfere with the reference signal. At this time, it is necessary to control the shift amounts of the phase π/4 and −π/4 accurately.
Japanese Laid-Open Patent Publication No. 2007-181171 (Document 1) discloses a related art that separates the electrical output signal converted from the optical phase modulated signal and monitors the eye aperture. This makes it possible to control the shift amount of the phase to be π/4 and −π/4 by controlling the shift amount of the phase so that the eye aperture becomes the maximum.
Japanese Laid-Open Patent Publication No. 2007-20138 (Document 2) discloses a related art that multiplies an I-phase electrical signal, which is converted from the optical phase modulated signal, before inputted into the data reproducing circuit by a Q-phase electrical signal after outputted from the data reproducing signal, and that controls the shift amount of the phase based on the result of the multiplication.